In the semiconductor fabrication technology, it is necessary to selectively plate certain areas of a substrate, particularly, in high level packages upon which integrated circuit chips are to be mounted thereon. One example of such high level packages is a multilayer ceramic module which, typically, provides the necessary interchip connections as well as the redistribution of I/Os to enable the connections to cards, boards, and the like. Substrates of this type are built with green ceramic sheets, such as those known in the industry as 9211, having connecting patterns that provide the internal interconnection metallurgy system, and which have been punched with a plurality of via holes. Whereas interconnecting lines and patterns are made of standard conductive materials, such as Al, the metallurgy of the via holes is typically made of screened refractory metals, such as W, Mo, Ta, Ti, Mn, and the like, to allow co-firing of the ceramic and the metallurgy. These materials are capable of withstanding the high temperatures necessary for sintering the green sheets forming the ceramic substrate.
Prior to sintering, the green sheets are stacked on top of each other, such that via holes filled with conductive material, (preferably, with refractory metal) in one sheet match corresponding vias in another. Ultimately, an interconnecting or redistribution line in one sheet will find its way to the top (or bottom) surface with the help of the interconnected vias. The exposed screened and sintered refractory metal area at the uppermost (or lowermost) layer must then be capped with metal that can be brazed or soldered to allow affixing module pins (if at the bottom layer) or to make the necessary connections with C4 balls or solder bumps of chips mounted on the substrate. Vias filled with W, Mo, and the like, are not wettable with braze or solder, hence the metal requires a layer of metal that can be soldered or brazed on the surface for further interconnection to the card or module.
The plating process typically includes a plating bath that contains ions of the metal to be deposited along with an auto-catalytic chemical reducing agent for the metal ions, and deposits the metal in a layer by reducing the metal ions within the solution. Typical reducing agents include hypophosphite ions, most commonly used for nickel chloride in an acid solution or amine boron as a reducing agent. The nickel layer will then contain traces of phosphorous in the former example, and boron in the latter. These residues have an adverse effect in that they can cause reliability problems by introducing stresses within the metal layer and severely impact the wettability of the surface, negatively impacting the ulterior brazing or soldering steps that follow.
Plated Ni metallurgy on the refractory metal is one way of making a solderable/brazable surface. Ni can be plated in two ways, either by electroplating or by electroless plating. Electroless plating has the advantage that not all the features need to be electrically connected in order to perform the plating. Plating from an electroless bath tends to incorporate some of the Ni complex into the film, resulting in a film that is not pure. Following plating, a thermal diffusion step is necessary to provide adhesion to the refractory metal. The impurity of the Ni film precipitates out during the diffusion step as Ni.sub.3 P, leading to a major problem with the properties of the film. The plated film has high residual stresses, a potential cause of micro-cracks which negatively affect the hermeticity of the package. These precipitates also cause wettability problems. Subsequent cleaning steps tend to remove the Ni, leaving behind Ni.sub.3 P precipitates, that result in the formation of "black vias" that are prone to wettability problems thereinafter.
Another bath that can also be advantageously used for electroless plating is one containing a Ni amine-boron complex. This bath co-deposits some of the boron alongside the Ni. Such a bath has several advantages over the hypophosphite previously described. The Ni diffusion which promotes adhesion between the refractory metal results in a low stress film which solves the micro-cracking problem. Boron precipitates as Ni.sub.3 B at about 400.degree. C. during the early stages of the diffusion. However, at higher temperatures, Ni.sub.3 B dissociates and the boron diffuses into the glass in the underlying feature. Glass is required, as will be described hereinafter, to eliminate boron from the Ni film. This dissociation at higher temperatures leads to a clean film made of Ni only, with a substantial improvement in its wettability characteristics over a film produced using the hypophosphite bath described above.
The use of glass for eliminating boron from the Ni film and for improving the quality of electrolessly deposited layer of Ni is described by Fleming et al. in U.S. Pat. No. 4,407,860. More particularly, a bath containing an amine-boron autocatalytic reducing agent on a surface containing glass is provided, and the boron containing Ni layer is subsequently heated to a temperature of at least 750.degree. C., long enough to make it possible for the boron to diffuse from the layer into the glass in the substrate.
Referring to FIGS. 1a-1c, there are shown a sequence of metallographic cross-sections of a prior art diffusion nickel film deposited on a molybdenum paste containing glass. Structures of this type are typically diffused at a temperature of 860.degree. C. for 15 minutes.
Initially, a Ni layer is deposited on the surface of a sintered alumina substrate 10. Various patterns are selectively formed into a configuration that is defined either by an underlaying metallurgy pattern, (usually, a screened pattern of the refractory metal), or by activating the area by means of an agent typical of an electroless plating process. The use of the electroless plating technique makes it possible to selectively plate only screened metal areas. The areas to be plated must have glass particles 12 adjacent to the surface, normally ranging from 0.5-15% by weight. The base used for the deposition of the electroless Ni layer 17 includes a screened layer formed of a refractory metal 11, such as W, Mo, Ta, Ti, Mn and glass fruit 12 in an amount ranging from 4-11% by weight. The pattern can be screened on a green sheet and sintered alongside the substrate or, alternatively, it can be screened and sintered on an previously sintered ceramic substrate. In an alternate process, a metallic screened pattern may be formed using a paste devoid of glass frit. However, even in this latter case, glass must be present in the substrate, in order that it migrate to infiltrate the paste pattern in an appropriate amount.
Prior art methodologies include activating the refractory metal surface by dipping the sintered alumina substrate in a bath of PdCl.sub.2 for a length of time sufficient to deposit a layer of Pd seed metal on the refractory surface. Following the cleaning process, the substrate is placed in a nickel plating bath (FIG. 1b) containing an amino boron auto catalytic reducing agent and Ni ions to plate a nickel film 17 of desired thickness on the metal features. Preferably, the Ni layer is deposited only on the underlying refractory metal areas. Typically, the Ni layer will contain 0.1-0.7% of boron by weight, leading to a layer having a thickness ranging from 2-15 .mu.m.
The substrate processed in this manner is then heated in an inert atmosphere at a temperature ranging from 750.degree. to 1200.degree. C. The environment can be non-reactive gas, such as He, or Ar, and a reducing atmosphere that includes either H.sub.2, a combination of H.sub.2 and N.sub.2, or a vacuum. The heating step is carried for a time such that it causes the boron in the Ni layer to diffuse downward where it may react or even be held by the glass in the underlying Mo layer. The step of diffusing the plated nickel film produces adhesion to the refractory metal. The above diffusion step produces, initially, a film with Ni.sub.3 B precipitates 13. These precipitates are undesirable since they affect the properties of the film and of the solder/braze wetting. Diffusion at high temperatures, i.e., 750.degree. C. and above, enhances the film properties and eliminates boron precipitates from the film by diffusing the boron into the underlying glass inclusions.
Prior art processes suffer from certain serious limitations, not the least in that they are overly restrictive. The step of providing a substrate that includes glass particles adjacent to the surface areas that receives the metal layer is deemed unnecessary. Further, the temperatures commonly used by the prior art techniques are unnecessarily high and add to the cost of manufacturing.